I have a '72 PX-10 with the original Simplex seat post and I experience the same issue when I've swapped out saddles. It's kind of a pain, but the rails will spread enough to install. As a matter of fact, I just swapped out a saddle yesterday, changing out a red Flite that I had been riding for a period correct Ideale 80. You would think the French saddle would fit right on, right? No way! Same problem, so I had to very carefully slide the darned thing on. Crazy, but you have my empathy!